The present invention aims to develop an anticorrosion coating for metal workpieces, free of chromium and chromium-based derivatives, and in particular of hexavalent chromium, which can preferably be applied in thin layers while at the same time providing highly effective protection.
The invention applies to workpieces of a metallic nature (in particular steel, cast iron) which require good resistance to corrosion, with a thin layer. The geometry of the workpieces has little importance as long as they remain suitable for application using the processes described.
The invention makes it possible to improve the anticorrosion properties of the workpieces treated without using chromium, and in particular hexavalent chromium, in the formulation of the coatings.
Many chromium-based anticorrosion treatment solutions will have been proposed to date. While they are generally satisfactory in terms of protecting the metal workpieces, they are, however, increasingly criticized due to their consequences in terms of toxicity, and in particular due to the harmful consequences to the environment.
The present invention relates to an anticorrosion coating for metal workpieces, characterized in that it consists of the synergistic combination of a first layer based on metallic zinc and one or more coating layer(s) obtained using an aqueous composition containing a particulate metal, a suitable solvent, a thickener and a binder consisting of a silane.
The first layer based on metallic zinc may, for example, advantageously be produced by mechanical deposition. Such a layer, consisting of zinc or more generally of a zinc and iron alloy, or even of a mixture of particles of zinc and of iron, may be applied in the context of the present invention in an amount of between 50 and 300 mg/dm2 of coating metal. For some particular applications, it may even be possible to use smaller deposited amounts.
Advantageously, the layer based on metallic zinc is mechanically deposited by a shotblasting operation with the aid of steel shot having at least one outer layer comprising either pure zinc or a zinc-based alloy.
Such mechanical deposition of a layer based on metallic zinc may also be carried out by a shotblasting operation with the aid of a mixture of steel shot and shot consisting of a steel core and having, on the surface, at least one outer layer based on a zinc alloy or an outer layer of pure zinc.
Finally, this mechanical deposition of the layer based on metallic zinc may also be obtained by shotblasting with the aid of shot based essentially on an iron alloy, the blasting being performed in the presence of a zinc powder or zinc granules, which is therefore applied due to the mechanical effect of the blasting.
The term xe2x80x9cshotxe2x80x9d or xe2x80x9cmicroshotxe2x80x9d used in the context of the present invention to describe the shotblasting operations should be understood in the broad sense, i.e. understood to encompass all types of shapes of particles or microparticles to be blasted on to the surface of the workpieces.